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Is it possible to have a variable interval within a javascript function?

I've been trying without success to set up a way of varying the interval used when incrementing a value by one. It's set to increment every 9 seconds but I'd like the counter to look a little less robotic and instead increment by a repeated variation of numbers, for example, 3 seconds, 7 seconds, 12 seconds, 10 seconds and 13 seconds (the five numbers add up to 45 to ensure an average of 9 seconds is maintained).
I've tried to put these numbers into an array and loop the value of 'interval' through them but I've now realised that value can't be changed within the context of the function once it's started.
Would be super grateful for any advice here. Thanks!
Current code for more 'robotic' count:
let interval = 9000;
let shiftCounter = {{ row.total }};
window.setInterval(function () {
document.getElementById("shiftsCreated").innerHTML = shiftCounter.toLocaleString('en');
shiftCounter = shiftCounter + 1;
}, interval);

You can use setTimeout instead, and every time it completes, call new timeout by choosing random delay or whatever order you want.
let counter = 0;
const intervals = [3, 7, 10, 12]
increment(0);
function increment(timeout) {
setTimeout(() => {
console.log(`Counter: ${counter}.`)
counter++;
increment(intervals[Math.floor(Math.random() * intervals.length)] * 1000)
}, timeout);
}

Creating a Countdown with JavaScript

I am trying to make a Countdown that just log in the terminal the seconds remaining. It should be an interval of 1 second between every log. It has to be as a method of an object. I don´t know if it's because of the scope but somehow it's not working after many trials and code-rewriting.
Anybody can help me with this?
var Countdown = function(seconds) {
this.counter = seconds;
this.start = function() {
setTimeout(
function() {
while (this.counter >= 0) {
console.log(this.counter);
this.counter -= 1;
}
},1000)
}
}

I would use a setInterval() for a simple countdown timer. I would also write my function for the math loop outside of the setInterval and then call on the function within the interval, like the following => setInterval(timerFunction(), 1000). There is no need for a loop when you use the interval, it does the looping each defined time increment as set in your setInterval(). Each time the interval fires, the function will do its thing.
EDIT: added a conditional to see if the interval time has run out.
I have included an example of a simple count down timer in my answer below along with notation inside the code that helps to explain further. Hope this helps.
Also, by terminal, I assume you mean the console? My example displays the setInterval in the console...
let sMin = 2; // amount of minutes you wish to start with
let time = sMin * 60; // format for minutes by multiplying by 60 as we have 60 seconds in each minute
let countdown = setInterval(update, 1000) // set up a setInterval for the countdown function
// create a function that will countdown the seconds
function update() {
// define our minutes using Math.floor(time / 60)
let min = Math.floor(time / 60);
// define our seconds by modulating time with 60, our seconds units
let sec = time % 60;
// tenerary conditional to see if seconds is set to 0 for proper display of formatting as seconds
sec = sec < 10 ? '0' + sec : sec;
// display the countdown timer in time format using the minutes and seconds variables
console.log(`${min}:${sec}`);
// decrement time by one second with each interval as set in the setInterval call `1000`
time--;
// clear the interval if the minutes and seconds are both set to 0
min == 0 && sec == 0 ? clearInterval(countdown) : countdown;
}

Yes it was because of the scope. As you are using this inside setTimeout() it uses the global scope.
var Countdown = function(seconds) {
this.counter = seconds;
this.start = function() {
setTimeout(
function() {
while (this.counter >= 0) {
console.log(this.counter);
this.counter -= 1;
}
}.bind(this),1000)
}
}
I'm using bind to set "this" to current context.
And with reference to your question about timeout, instead of using setTimeout() use setInterval() to achieve your need about log with respect to seconds

function countDown(whileCountingDown, forHowLong, whenFinishedThen){
//error handling begin(for user's understanding)
if(arguments.length<3){return RangeError("ALL three arguments must be used")}
var typeErrors=[]
if(typeof whileCountingDown!="function"){typeErrors.push("whileCountingDown MUST be a function")}
if(typeof forHowLong!="number"){typeErrors.push("forHowLong MUST be a number(and it represents seconds)")}
if(typeof whenFinishedThen!="function"){typeErrors.push("whenFinishedThen MUST be a function")}
if(typeErrors.length>0){return TypeError(`There are ${typeErrors.length} parameters that are incorrect data types\n\n${typeErrors.join('\n')}`)}
//error handling begin(for user's understanding)
//........................................................................................................................
//the part that matters to you once you enter correct stuff
var i=setInterval(()=>{forHowLong--
if(forHowLong<=0){//count finished, determine what happens next
clearInterval(i); whenFinishedThen()
}
else{whileCountingDown(forHowLong)}//do this for each second of countdown
},1000)
}
console.log("The timers in the console and on the html element are 2 DIFFERENT countdowns")
//example use
countDown((m)=>{console.log(`${m} seconds left`)}, 30, ()=>{console.log('Cheers, the countdown is OVER')})
//obviously you can do stuff like edit randomHTML-Element.innerText for each second of the countdown or anything as you so desire since what i made is kindof flexible
//..........................................................................................................................
//more examples
//now for some fun stuff, we're gonna be editing an html structure, but first, we're going to make a 'timeParse' function to make it look kind of elegant
function timeParse(seconds){
//yup, error handling begin
if(typeof seconds!="number"){return TypeError("The parameter 'seconds' MUST be a number")}
//yup, error handling end
//below is the stuff to look at
var timeArr=[seconds]
if(timeArr[0]>=60){//if seconds >= 1 minute
timeArr.unshift(Math.floor(timeArr[0]/60))
timeArr[1]=timeArr[1]%60
if(timeArr[0]>=60){//if minutes >= 1 hour
timeArr.unshift(Math.floor(timeArr[0]/60))
timeArr[1]=timeArr[1]%60
if(timeArr[0]>=24){//if hours >= 1 day
timeArr.unshift(`${Math.floor(timeArr[0]/24)}d`)
timeArr[1]=timeArr[1]%24
}
}
}
return(timeArr.join`:`)
}
//now for applying countDown to things other than the console
function countDownAgain(){//just something that will show the flexibility of what i made.. im going above and beyond because i wanna look back on my answers as notes on how to do things(also ez copy pasting for future me xD)
countDown(
(s)=>{document.getElementById('toTime').innerText="Second Count "+timeParse(s)},
86401,
()=>{document.getElementById('toTime').innerText="No way you waited that long LOL"}
)
}
countDown(
(s)=>{document.getElementById('toTime').innerText="First Count "+timeParse(s)},
100,
countDownAgain
)
<div style="background-color:red;height:100px;text-align:center;line-height:50px"><h1 id="toTime">Count Down Time :}</h1></div>

Javascript Self Adjusting Count-Down Timer

I am trying to create an accurate countdown timer (accurate to the second), so that the second timer changes every second as opposed to lagging behind due to some other code I have.
I have a very limited understanding of how this works and I have attempted the following:
var seccount=100;
var seccounter=setInterval(sectimer, 1000);
var start = new Date().getTime();
function sectimer() {
seccount--;
elapsed = Math.ceil(seccount/100);
var diff = (new Date().getTime() - start) + seccount;
window.setInterval(sectimer, (0 + diff));
if (seccount < 0) {
return;
}
document.getElementById("sectimer").innerHTML=seccount+" seconds left!"
}
However, this timer seems to skip a few numbers some times. Is there any way to fix this? What am I missing?
EDIT:
Any count-down timer that is more accurate will do. I have been experimenting with the following timer that I have to try and make it more accurate, but I am not having much luck. Basically, it counts down every 1.1 seconds about (instead of every 1 second),
var seccount=100;
var seccounter=setInterval(sectimer, 1000);
function sectimer() {
seccount--;
if (seccount < 0) {
return;
}
document.getElementById("sectimer").innerHTML=seccount+" seconds left!";
}

setInterval Speed Up | Javascript

I'm trying to gradually depreciate the interval in which the function inside a setinterval function is executed
Here's abstracted code for principle:
var speed = 100;
window.setInterval( speed-= 0.01 , speed);
Am I correct in believing the value for speed is taken once, on the first execution of setInterval and used, instead of being taken at every execution.
If so, how can I get around this? I verified this by setting the interval to 1000 and setting the "speed-=" to 999, and then printing the value for speed.
The value jumped from 1000 to 1 but then kept going down by 999, and even after the value became negative the functions inside setinterval were executed every 1 second.

An example of recursive call of setTimeout with changing interval.
function go() {
var node = document.createElement('div');
node.innerHTML = speed.toFixed(2);
document.body.appendChild(node);
if (speed > 1) {
setTimeout(go, speed);
}
speed *= 0.9;
}
var speed = 100;
go();

How to produce a time delay in JavaScript for a chat App?

I'm currently a beginner at JavaScript and predominantly code in Java.
My Question is is regarding invoking the document.write("string here") periodically (every 1/2 seconds) to append any new unprinted chat message to the client. I earlier tried using the following code :
<html>
<body onload="yantra();">
<script type="text/javascript">
x = 0;
function yantra(){
document.write("Hello<br>");
i = 1;
for(i = 0; i < 100; i++){
setTimeout("writeOneNum()", 1000);
}
}
function writeOneNum(){
x =x + 1;
document.write(x+"<br>");
}
function blank(){}
</script>
</body>
</html>
Instead of it printing 1 to 100 every 1000 millisecond as I expected it to print; in actuality, it printed 1 to 100 at one go (meaning without any delay).

Well, you are setting all the timeouts at once, so of course it fires them all at once. Your code will work with small modification to the loop:
function yantra(){
document.write("Hello<br>");
i = 1;
for(i = 0; i < 100; i++){
setTimeout("writeOneNum()", 1000 * i);
}
}
By multiplying the time with i, the first is fired instantly, the second at 1000 * 1 = 1000 ms, the third at 1000 * 2 = 2000 ms etc.
In your case, it could be wiser to consider the setInterval function:
function yantra(){
document.write("Hello<br>");
setInterval("writeOneNum()", 1000);
}
That will fire the writeOneNum() every second infinitely. To stop the interval at some point, take a look at clearInterval on that same link above.